Mass Balance Equation
The default node attributed to the Transport of Diluted Species interface models chemical species transport through diffusion and convection and solves the mass conservation equation for one or more chemical species i:
(6-13)
Equation 6-13 in its form above includes the transport mechanisms diffusion and convection. If Migration in Electric Field is activated (only available in some add-on products), the migration mechanism will be added to the equation as well. See more details in the section Adding Transport Through Migration.
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ci is the concentration of the species (SI unit: mol/m3)
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Di denotes the diffusion coefficient (SI unit: m2/s)
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Ri is a reaction rate expression for the species (SI unit: mol/(m3·s))
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 u is the mass averaged velocity vector (SI unit: m/s)
The mass flux diffusive flux vector
The mass flux relative to the mass averaged velocity, Ji (SI unit: mol/(m2·s)), is associated with the mass balance equation above and used in boundary conditions and flux computations. The Transport of Diluted Species interface always includes mass transport due to molecular diffusion. In this case the mass flux Ji defines the diffusive flux vector
(6-14)
An input field for the diffusion coefficient is available. Anisotropic diffusion tensor input is supported.
When Migration in Electric Fields is activated, the migration term is also added to the diffusive flux vector as shown in the section Adding Transport Through Migration.
The third term on the left side of Equation 6-13 describes the convective transport due to a velocity field u. This field can be expressed analytically or obtained from coupling the physics interface to one that solves for fluid flow, such as Laminar Flow. Note that all fluid flow interfaces solve for the mass averaged velocity.
On the right-hand side of the mass balance equation (Equation 6-13), Ri represents a source or sink term, typically due to a chemical reaction or desorption on a porous matrix. To specify Ri, another node must be added to the Transport of Diluted Species interface — the Reaction node for example, which includes an input field for specifying a reaction expression using the variable names of all participating species.